U.S. patent number 5,764,402 [Application Number 08/737,011] was granted by the patent office on 1998-06-09 for optical cell control system.
This patent grant is currently assigned to Glaverbel. Invention is credited to Jean-Fran.cedilla.ois Thomas, Pierre Vezin.
United States Patent |
5,764,402 |
Thomas , et al. |
June 9, 1998 |
Optical cell control system
Abstract
A control system for an optical cell (light valve) is provided
which comprises a first (oscillator) circuit supplied by a low
voltage power source and including a primary winding of an
induction coil and a secondary (resonant) circuit which includes
the optical cell and a secondary winding of the induction coil. The
secondary circuit includes the inductance of the secondary winding
and the optical cell, and the induction coil provides a weak
coupling between the primary and secondary windings. The resonant
circuit provides a large over-voltage coefficient and great
stability and the configuration made possible by the invention
facilitates a significant reduction in the bulk of the control
system.
Inventors: |
Thomas; Jean-Fran.cedilla.ois
(Braine-le-Chateau, BE), Vezin; Pierre (Bures/Yvette,
FR) |
Assignee: |
Glaverbel (Brussels,
BE)
|
Family
ID: |
10754380 |
Appl.
No.: |
08/737,011 |
Filed: |
October 28, 1996 |
PCT
Filed: |
April 24, 1995 |
PCT No.: |
PCT/BE95/00040 |
371
Date: |
October 28, 1996 |
102(e)
Date: |
October 28, 1996 |
PCT
Pub. No.: |
WO95/30172 |
PCT
Pub. Date: |
November 09, 1995 |
Foreign Application Priority Data
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|
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Apr 29, 1994 [GB] |
|
|
9408603 |
|
Current U.S.
Class: |
359/272; 359/265;
349/33; 310/321; 359/268; 359/245; 359/275 |
Current CPC
Class: |
G02F
1/13306 (20130101); G02F 1/163 (20130101); A61F
9/023 (20130101) |
Current International
Class: |
A61F
9/02 (20060101); G02F 1/01 (20060101); G02F
1/03 (20060101); G02F 1/17 (20060101); G02F
001/03 (); G02F 001/15 (); H01L 041/04 () |
Field of
Search: |
;359/265-275,245
;345/87,98,105,204,211 ;310/317,321 ;349/33 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2366958 |
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May 1978 |
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FR |
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2190516 |
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Nov 1987 |
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GB |
|
90/07381 |
|
Jul 1990 |
|
WO |
|
Primary Examiner: Nelms; David C.
Assistant Examiner: Lester; Evelyn A.
Attorney, Agent or Firm: Spencer & Frank
Claims
We claim:
1. A control system of the reflexivity or transmissivity of an
optical cell (14), which system comprises a first circuit (1)
supplied by a low voltage power source (3) and including an
oscillator (4) and a primary winding (11) of an induction coil and
further comprises a secondary circuit (2) which includes the
optical cell (14) and a secondary winding (12) of the
aforementioned induction coil, the secondary circuit (2) being a
resonant circuit, characterised in that said secondary circuit (2)
includes the inductance of the secondary winding (12) and in that
the magnetic circuit of the induction coil comprises a magnetic
resistance to the passage of magnetic flux, to provide a weak
coupling between the primary (11) and secondary (12) windings.
2. A control system as claimed in claim 1, in which the optical
cell (14) is connected directly to the secondary winding (12) and
the induction coil provides substantially all of the inductance of
the resonant circuit (2).
3. A control system as claimed in claim 1, in which the induction
coil comprises a magnetic core (10) which provides a magnetic
resistance to the passage of flux in the magnetic circuit which it
forms.
4. A control system as claimed in claim 3, in which the magnetic
core (10) includes a gap (13) in the path of the lines of magnetic
flux.
5. A control system as claimed in claim 4, in which the gap (13) in
the magnetic core is at least 0.1 mm.
6. A control system as claimed in claim 5, in which the gap (13) in
the magnetic core is at least 0.2 mm.
7. A control system as claimed in claim 1, in which the primary
(11) and secondary (12) windings of the coil do not overlap each
other.
8. A control system as claimed in claim 7, in which the primary
winding (11) is formed around one portion of a magnetic core (10)
and the secondary winding (12) is formed around another portion of
the magnetic core (10).
9. A control system as claimed in claim 1, in which the primary
winding (11) comprises less than 100 turns.
10. A control system as claimed in claim 9, in which the primary
winding (11) comprises 10 to 80 turns.
11. A control system as claimed in claim 10, in which the primary
winding (11) comprises 40 to 80 turns.
12. A control system as claimed in claim 1, in which the secondary
winding (12) comprises 140 to 300 turns.
13. A control system as claimed in claim 1, in which the coupling
coefficient of the induction coil is less than 0.7.
14. A control system as claimed in claim 13, in which the coupling
coefficient of the induction coil is less than 0.5.
15. A control system as claimed in claim 1, in which the voltage
across the optical cell (14) is adjusted by modulating the size of
energy impulses applied to the primary winding (11).
16. A control system as claimed in claim 1, which includes at least
one feedback line (5') from the secondary circuit (2) to the first
circuit (1).
17. A control system as claimed in claim 16, in which the or a
feedback line (5') regulates the voltage in the secondary circuit
(2) to ensure at all times the required voltage across the optical
cell (14) for a required level of darkening of the cell (14).
18. A control system as claimed in claim 16, in which the or a
feedback line (5') regulates the frequency acting on the oscillator
(4) frequency in the first circuit (1) to ensure at all times the
operational frequency of the secondary circuit (2) at the resonant
frequency thereof.
19. A control system as claimed in claim 1, in which the secondary
circuit (2) includes at least one capacitor (16) in parallel with
the optical cell (14).
20. A control system as claimed in claim 19, in which the secondary
circuit (2) includes, in parallel with the optical cell (14), two
or more capacitors (16) arranged in series with each other.
21. A control system as claimed in claim 1, in which the resonance
of the secondary circuit (2) establishes the oscillation in the
first circuit (1) and thereby determines the operational frequency
of the system.
22. A control system as claimed in claim 1, which is regulated by
at least one photo-sensitive optical device (32, 33) which detects
the incident light falling on the optical cell (14).
23. A control system as claimed in claim 22, which includes two
photo-sensitive optical devices (32, 33), one positioned to monitor
potentially dazzling light coming from behind a vehicle and the
other to monitor an ambient light level.
24. A control system as claimed in claim 1, in which the optical
cell (14) is of the type which incorporates a fluid suspension of
dispersed minute particles capable of orientation by an electrical
field to change the transmission of light through the
suspension.
25. A control system as claimed in claim 1, in which the optical
cell is part of a rear-view mirror of a motor vehicle.
26. A control system as claimed in claim 25, which is located
within a housing of the rear-view mirror.
27. A control system as claimed in claim 25, in which the induction
coil comprises a core (10) which is sized to fit within a housing
of the rear-view mirror.
Description
This invention relates to a control system for an optical cell
(also called a light valve).
An optical cell may be formed by sandwiching a layer of sensitive
material between two parallel plates of rigid, generally
transparent sheet material, each plate having an electrically
conducting surface layer facing the sensitive material. Examples of
constituents of the sensitive material include suspended particles,
liquid crystals and a layer of electrochromic material.
By applying or not applying an electric potential across the facing
plates the constituents can be switched between a state in which
light can pass and a state in which light is absorbed, scattered or
reflected.
Optical cells, including optical cells having a control system
according to the invention, can be used in mirrors or in glazing
panels for vehicles or buildings so as to provide variable light
transmission. For example they can be used as a glazing panel
adjustable between an opaque and clear condition to limit solar
transmission or to conceal the interior of a room or vehicle to
provide privacy therein. They can be used in a vehicle sun-visor or
sunshine roof panel or on an aircraft porthole.
The control system of the invention is particularly well suited for
use with an optical cell used in a rear-view mirror of a motor
vehicle, and is described herein largely with reference to that
application.
The use of an optical cell in a rear view mirror is known for
example from French patent specification FR 2 366 958 (Brisard
Gerard) so as to provide a rear-view device in which the
reflectivity varies as a function of the degree of dazzle.
Traditional vehicle rear-view mirrors with an anti-dazzle feature,
often called "prismatic" mirrors, have a "day" position and a
"night" position, the mirror position being changed manually by the
vehicle driver between the day and night positions. In the day
position the degree of luminous reflectivity from a rear-view
mirror is required to be high, generally greater than 50%. In the
night position the reflectivity is limited to 12% or less, often
about 4%, in order to avoid dazzling of the driver by fights such
as the headlights of following vehicles.
Optical cells offer a rear-view mirror with the possibility of
automatic adjustment according to the incident light conditions,
adjusting from day to night positions and vice versa and to
intermediate variations between those limits, thereby giving
advantages in terms of convenience and safety. The cell is located
adjacent and parallel to the reflective surface of the mirror and
in the line of sight between the reflective surface and the vehicle
driver. The cell thus provides a variable level of light reflection
from the mirror to the driver. In one preferred configuration the
reflective layer is provided by one of the electrically conducting
layers being a material which is also reflective.
The level of light transmission or reflection through the optical
cell is adjusted by the control system, which is activated by the
external conditions. The control system is desirably housed
alongside the optical cell so as to form a combined unit therewith.
Traditional control systems have however been of a bulk which has
made for difficulties in miniaturising any unit of which they have
formed a part.
An object of the present invention is to provide a control system
for an optical cell which can be conveniently accommodated adjacent
to the cell.
According to the invention there is provided a control system for
an optical cell, which system comprises a first circuit supplied by
a low voltage power source and including an oscillator and a
primary winding of an induction coil and further comprises a
secondary circuit which includes the optical cell and a secondary
winding of the aforementioned induction coil, characterised in that
the secondary circuit is a resonant circuit which includes the
inductance of the secondary winding and the optical cell, and the
induction coil provides a weak coupling between the primary and
secondary windings.
The control system of the invention thus uses the inductance of the
secondary winding of the coil as the inductance of a resonant
(oscillating) circuit. The configuration made possible by the
invention facilitates a significant reduction in the bulk of the
control system
A particular advantage of the system according to the invention is
that high voltages are confined to the secondary circuit, thereby
providing a system with a reduced number of components subjected to
high voltages. Since high voltages can create problems of safety
and electromagnetic interference, the limitation of high voltage to
the secondary circuit is beneficial in reducing the space occupied
by high voltage components and in reducing the amount of protective
casing required to enclose them.
Several different types of optical cell are known. These include an
electrochromic optical cell or a liquid crystal optical cell or an
electrodeposition optical cell. In electrodeposition the passage of
a current through a transparent liquid containing a metallic salt
causes the migration of metallic ions to the surface of the glass
and the formation of a metallic coating which absorbs the light.
The electrodes in this case are SnO.sub.2 coatings. Liquid crystal
optical cells, electrodeposition optical cells and electrochromic
optical cells are generally transparent at rest but in certain
conditions, such as the presence of an over-voltage or a prolonged
period in an excited state, the return to a state of clarity from
an excited state may take some time, even a matter of hours, and
thus the switching speed of the cell may be relatively slow.
The preferred type of optical cell for use with the control system
of the invention incorporates a fluid suspension of dispersed
minute particles capable of orientation by an electrical field to
change the transmission of light through the suspension, such as
described, for example, in United States patent U.S. Pat. No.
3,655,267 (Research Frontiers). These optical cells switch rapidly
from a clear state to a dark state. They also provide a wide range
of luminosity. Fluid suspensions of herapathite in a suitable
liquid such as iso-pentyl acetate are preferred, although other
types of particles can be used, such as graphite, mica, garnet red,
aluminium and periodides of alkaloid sulphate salts.
The plates of transparent material forming an optical cell are
typically located at a substantially uniform distance of about 50
.mu.m from each other across the whole of their facing areas. If
this distance is not maintained within a tolerance of, for example,
about 5 to 10 .mu.m the transparency of the cell is not uniform and
problems may also arise in short circuiting of the electrical field
between two adjacent points on the opposing faces. This
uniform-distance requirement imposes certain limits on the material
from which the plates can be formed. Thus although plastic
materials such as polyethylene terephthalate can be considered,
difficulties may occur in maintaining the constant spacing between
the sheets of plastics material over the whole surface of the cell.
In general it is therefore preferred to employ glass sheets.
The faces of the plates facing each other in the cell are coated
with an electrically conducting material. The preferred coating
material is indium tin oxide (ITO), which is both conductive and
transparent. The mirror surface of the rear view mirror of which
the cell may form a part can conveniently be provided by a
reflective layer of the face of one of the cell plates opposite to
the electrically-coated face. The material for any such reflective
coating on the plates is usually silver, chromium or aluminium.
When the cell forms part of a vehicle rear-view mirror, the
reflective mirror surface and the cell are located in a housing
attached to the vehicle, for example on the vehicle windscreen or a
door. The circuit for controlling the adjustment of the optical
cell is positioned in or on the housing. Electrical connectors are
also provided in or on the housing to connect the optical cell to
the electrical system of the vehicle.
In the control system according to the invention the optical cell
is preferably connected directly to the secondary winding and the
induction coil provides substantially all of the inductance of the
secondary (resonant) circuit. This parallel resonant circuit
provides a greater over-voltage coefficient and greater stability
than a series circuit. The fact that the induction coil provides
substantially the whole of the inductance of the resonant circuit
ensures a slight bulk for the control system.
The term "weak coupling" is used herein, with reference to the
magnetic coupling between the primary and secondary coil windings,
to mean a coupling akin to that of a transformer but differing in
that inductance leakage is deliberately increased. The coupling
coefficient K can be calculated by the formula: ##EQU1##
in which L.sub.p is the inductance of the primary winding, L.sub.s
is the inductance of the secondary winding and M is the mutual
inductance. For the purposes of the invention the coil should
preferably have a coupling coefficient of less than 0.7, most
preferably less than 0.5.
The coupling should be weak so as to reduce the influence of the
primary circuit on the impedance of the secondary circuit, while
being sufficient to transfer the energy necessary to initiate and
maintain the resonance in the secondary circuit. Thus the energy is
introduced by the bias of the coil, while avoiding disturbance to
the characteristics of the secondary circuit
The coil according to the invention is thus not constructed as a
true transformer, in which as strong a coupling as possible is
generally required, and instead functions as a poor transformer.
The "weak coupling" coil of the invention does not have the purpose
of a true transformer to transfer energy with the smallest possible
losses.
The magnetic core of the induction coil is preferably constructed
to provide a magnetic resistance to the passage of flux in the
magnetic circuit which it forms. This is conveniently achieved by
including a gap in the path of the lines of magnetic flux through
the magnetic core. The gap is formed of a non-magnetic material,
for example air or more usually a resin or plastic material. The
size of the gap in the magnetic core is preferably at least 0.1 mm,
most preferably at least 0.2 mm.
The secondary circuit provides the reactive energy to activate the
optical cell. The operational frequency of the system can be
imposed by the secondary circuit itself, in that the impulses in
the system are continuously and automatically adjusted to the
resonant frequency of the secondary circuit This is preferably
achieved by constructing the electrical circuit in such a way that
the secondary (resonant) circuit is an element which acts directly
on the oscillating circuit and thus itself imposes the operational
frequency.
In the case of a cell with a suspension of electrically orientable
particles the frequency is typically of the order of 8 to 25 kHz,
often in the range 16 to 25 kHz. The use of such alternating
current avoids migration of suspended particles across the narrow
distance between the adjacent plates, which migration would
adversely affect the uniform opacity or clarity required from the
cell. The frequency should be chosen to avoid audible
frequencies.
In a system in which the frequency is imposed by the oscillator in
the primary circuit, but not in the case of an auto-oscillating
circuit, the frequency must be initially adjusted to the resonant
frequency determined by the set-up of the cell circuit for each
cell.
The control system of the invention offers the advantage that only
a small amount of energy is needed to sustain the required
resonance. A specific further advantage of the system according to
the invention is that if the cell is broken the current may be
retained within the circuit but with a much lower voltage.
The coil comprises a conventional core, typically of soft iron. The
size of the core is preferably such as to fit within the housing of
a rear-view mirror including the optical cell, the core being
located behind the mirror relative to the vehicle driver.
The primary and secondary windings of the coil preferably do not
overlap each other. Thus the primary winding is preferably formed
around one portion of the core and the secondary winding around
another portion of the core. This non-overlapping configuration
also assists in providing the weak coupling between the windings
and is of further benefit in making the system sufficiently small
to fit into the mirror housing.
The primary winding preferably comprises less than 100 turns around
the core, more preferably 10 to 80 turns and most preferably 40 to
80 turns. The secondary winding typically comprises 140 to 300
turns. The winding (transformation) ratio is thus typically of the
order of 3 to 4:1. The main factor in achieving the required
voltage across the cell is, however, not the winding ratio but
rather the over-voltage in the secondary circuit. The said
over-voltage is a function of the capacity, the inductance and the
resistance of the elements which constitute the secondary circuit.
The ability to use the over-voltage in the secondary circuit to
achieve the required voltage across the cell is a particular
advantage of the invention.
The wave form in the secondary circuit is substantially sinusoidal,
even if the wave form of the impulses generated by the oscillator
is not. The conversion of a non-sinusoidal wave (for example a
square wave) generated by the oscillator into a sinusoidal wave in
the secondary circuit is facilitated by the weak magnetic coupling
in the induction coil. The voltage applied across the optical cell
can be controlled by adjustment of the quantity of energy emitted
by the oscillator in modulating the size of pulse, for example by
changing its duration, or alternatively can be controlled by
adjustment of the voltage peaks in the primary circuit.
The system preferably includes one or more feedback lines from the
secondary circuit to the first circuit. This offers the advantage
of adjusting the oscillator in response to the electrical
parameters found on the optical cell.
Thus a feedback line can be provided to regulate the voltage in the
secondary circuit and thereby to ensure at all times the required
voltage across the optical cell for a required level of darkening
of the cell. Alternatively or in addition the or a feedback line
can also regulate the frequency acting on the oscillator frequency
in the first circuit to ensure at all times the operational
frequency of the secondary circuit at the resonant frequency
thereof.
A reactive loop can be created which detects whether the
operational frequency of the secondary circuit is truly the
resonant frequency of the secondary circuit and sends any required
correcting signal to the oscillator in the primary circuit to
adjust its frequency so as to obtain resonance in the secondary
circuit.
If the operational frequency is not the same as the resonant
frequency of the secondary circuit, the over-voltage is lower and
the active energy consumption is increased. To obtain good
operations it is therefore advantageous to ensure that the
operational frequency is equal to the resonant frequency, although
control of the difference between the operational frequency and the
resonant frequency may also, to some extent, control the voltage
applied to the cell by controlling the over-voltage factor.
In one embodiment of the invention the secondary circuit includes
at least one capacitor in parallel with the optical cell. In
general it is preferred to employ two or more such capacitors in
series with each other. The use of capacitors in series has the
advantage of reducing the voltage applied across each individual
capacitor.
The control system of the invention is applicable to a variety of
different types of optical cell. The control system regulates the
provision to the optical cell of an alternating current supply. For
a cell with suspended orientable particles a voltage of up to about
125 V may be required, the voltage is applied between the
conductive surfaces of the cell to generate an electrical field
which orients the particles in a manner to allow the passage of
light. In order to vary the luminous reflectivity or transmissivity
of the optical cell, it is sufficient to vary the current voltage
applied to the optical cell. One may also vary the frequency, but
this is less efficient. The variation in luminosity is largely
proportional to the applied voltage, up to a saturation limit In
the absence of an electrical field, the particles are subject to
Brownian movement and thus restrict the passage of light through
the cell. In the presence of a weak field, the particles tend to
align with the field but continue to oscillate about their mean
position such that some light absorption light occurs. It is
necessary to reach a certain threshold value for the electrical
field, for example corresponding to a voltage of about 100 V, in
order for the particles to be substantially fully aligned in the
field and thus for minimum absorption of light to occur.
The control system is preferably regulated by at least one
photosensitive optical device which detects the incident light
falling on the optical cell. Advantageously, at least two such
light detection devices are employed, the first being positioned to
monitor potentially dazzling light coming from the rear of the
vehicle and the second being positioned to monitor the ambient
light level, for example the light coming through the windscreen,
light reflected by the roof of the vehicle or light diffused by a
transparent roof of the vehicle.
The control relies on the principle that a signal proportional to
the light level detected by the photo-sensitive optical device, or
on the difference in light levels detected by two such devices, is
employed to act upon the oscillator in the primary circuit so as to
adjust the voltage applied across the terminals of the optical
cell, and thus the opacity of the cell.
Where, in addition to an internal rear-view mirror, one or more
exterior rear-view mirrors are provided, the transmissivity and/or
the reflectivity characteristics of the external rear-view assembly
may be controlled by the same electronic circuit provided for the
control of the internal rear-view assembly, to provide simultaneous
adjustment of the transmissivity and/or reflectivity
characteristics. However, because of the scope for miniaturisation
and the small power consumption of control systems according to the
invention, it is possible to include a separate control system in
each of the mirrors. With such separate systems each of the mirrors
is thereby adjusted according to the specific light conditions
falling upon it.
The invention is further described below, by way of non-limiting
example, with reference to the accompanying drawings, in which:
FIG. 1 is a circuit diagram of one version of control system
according to the invention and intended for use in a motor
vehicle;
FIG. 2 is a sectional view of an induction coil used in the system;
and
FIG. 3 is a different sectional view of the FIG. 2 induction coil,
the section being taken along the line A--A' of FIG. 2.
The illustrated control system includes a first circuit 1
comprising a 12 volt DC battery 3, an oscillator 4 and a primary
winding 11 of an induction coil having a magnetic core 10. The
system further includes a secondary circuit 2 which includes the
secondary winding 12 of the induction coil, an optical cell 14 and
a capacitor 16 in parallel with the optical cell 14. The induction
coil is shown diagrammatically in FIG. 1 and in greater detail in
FIGS. 2 and 3.
The battery 3, which supplies low voltage power to the oscillator
4, is the electrical source for the whole of the electronic
circuit. Apart from the battery 3 there is provision for components
to impose negative and positive reference voltages upon certain
parts of the circuit The first of these components is a DC--DC
converter 3' in the line from the battery 3 to the oscillator
4.
The oscillator 4 has an associated actuation means 5, in this
instance simply a potentiometer, which serves to adjust the
frequency of the oscillator 4. In an alternative configuration the
actuation means 5 is replaced by a frequency feedback means 5'
(shown by dotted lines in FIG. 1) which detects the frequency in
the secondary circuit and adjusts the oscillator 4 to this
frequency. This alternative offers the benefit that the oscillator
frequency is automatically adjusted to be always at the frequency
of the secondary circuit
The optical cell 14 is of the type which incorporates a liquid
suspension of minute solid particles capable of orientation by an
electric field. The capacitor 16 is preferably formed by four
capacitors in series.
The system further includes a control circuit, indicated generally
by the numeral 30, which includes two photo-electric light
detection devices 32 and 33 linked to a detector control unit 34. A
reference voltage is applied to the unit 34. A detector signal line
35 leads from the unit 34 to a signal comparison unit 40. The
circuit 30 further includes an operational amplifier 36 with a
feedback circuit 39. A cell-operation detector line 37 leads from
the secondary circuit 2 to the comparison unit 40, from which a
signal line 38 leads to the amplifier 36.
In the illustrated system the comparison unit 40 is also provided
with an actuation means 41 (in this instance a potentiometer) to
set a threshhold voltage. The means 41, which is not an essential
component, serves to limit the voltage to a level sufficient for
the proper functioning of the mirror while not subjecting the cell
to a needlessly high voltage.
The configuration of the induction coil used in the present example
is shown in greater detail in the sectional views of FIGS. 2 and 3.
The magnetic core 10 is formed of two facing E-shaped ferrites with
a plastic spacer 13 between them. The spacer 13 provides resistance
to the passage of flux in the magnetic circuit of the core 10. The
secondary winding 12 is disposed around the central arm formed by
the opposing central bars of the E-shaped ferrites, whereas the
primary winding 11 is disposed around the opposing bars at one end
of the ferrites.
The control system as a whole is mounted in a housing (not shown)
and can be connected through the vehicle wiring harness to the 12
volt battery 3.
In use, the light detector 32 is positioned to detect the ambient
light conditions, for example by capturing light from the front of
the vehicle and/or light reflected by the roof, and the light
detector 33 is positioned to detect light from the rear of the
vehicle. The detector control unit 34 includes a difference
detector which compares the signals from the light detectors 32 and
33 and feeds to line 35 a signal proportional in strength to the
excess of the intensity of the rear-received light (33) over the
ambient light (32). Any dazzling light impinging on the detector 33
sends a corresponding signal to the difference detector. The line
37 carries a signal proportional to the voltage of the secondary
circuit 2. The signal sent by the control unit 34 via the line 35
is compared in the comparison unit 40 with the signal from the line
37 which indicates the secondary voltage and the comparison unit 40
in turn sends a control signal to the operational amplifier 36 via
the line 38. This control signal takes into account the level of
dazzle and the voltage actually applied across the optical cell. It
would similarly be possible to include a further feedback means to
provide a second control loop to feed back to the oscillator 4 the
established frequency of the cell 14 (as shown in dotted lines by
feedback means 5').
The signal from line 38 is amplified by the amplifier 36 to
stimulate the oscillator 4 to generate a pulsed wave (although a
sinusoidal wave or a square wave are possible variants) of low
voltage AC in the primary circuit 1. This wave in turn induces
through the coil a higher voltage in the secondary circuit 2 to be
applied across the cell 14. The voltage across the cell 14 is
further increased by the resonance in the secondary circuit and is
typically up to about 120 volts AC, thus permitting the application
to the cell 14 of a voltage to give partial or full alignment of
its suspended particles. Full alignment provides the maximum light
transmission through the cell 14 and thus the maximum reflectivity
of the rear-view mirror of which the cell 14 forms a part.
If the ambient light detector 32 observes good daylight or a high
level of artificial light and the rear-facing detector 33 observes
similar conditions then the difference between the respective
photo-electric signals is small and the unit 34 sends a signal via
lines 35 and 38 and the amplifier 36 to activate the oscillator 4
and to generate a cell-activating voltage in the secondary circuit
2. The comparison unit 40 is informed via line 37 of the voltage
actually produced in the secondary circuit and adjusts the command
signal sent via line 38 so as to obtain the maximum voltage across
the cell 14.
The oscillator 4 converts the 12 volt battery voltage to an AC
voltage and a voltage of 120 volts AC is achieved in the secondary
circuit 2. The AC frequency of the oscillator 4 is adjusted to the
resonant frequency of the secondary circuit 2 by external action
from the means 5 on the oscillator 4 and is normally about 20 kHz.
This adjustment can be easily achieved by measuring the active
current in the secondary circuit. When the frequency varies, the
current follows a curve which passes through a minimum. The
resonant frequency is achieved when the current is at the minimum.
This 120 volt maximum voltage in the secondary circuit 2 produces
full alignment of the suspended particles in the cell 14.
As an alternative the frequency feedback means 5' provides a
frequency control for the oscillator 4.
If the ambient light detector 32 observes dull or night-time
conditions and the rear-facing detector 33 observes similar
conditions then the difference between the respective
photo-electric signals is again small. The unit 34 again sends a
signal via lines 35 and 38 and, as described above, again achieves
the highest level of cell clarity and mirror reflectivity.
If, however, the ambient light detector 32 observes dull or
nighttime conditions and the rear-facing detector 33 observes a
dazzling full-beam headlight then the difference between the
respective photo-electric signals is large and the unit 34 sends a
corresponding signal to the amplifier 36. In this situation no
voltage is produced in the induction coil 11/12 and no voltage in
the cell 14. In the absence of a voltage in circuit 2 the cell
particles adopt a random disposition, rendering the cell opaque and
reducing the mirror reflectivity to its lowest level.
If the difference between the signals from the ambient light
detector 32 and the rear-facing detector 33 lies between the
extremes described above, for example in fairly dull conditions and
with a mildly dazzling beam through the rear window, the unit 34
sends a signal via the amplifier 36 to the oscillator 4 which gives
some stimulation of the oscillator 4 but the width of the pulses
generated in circuit 1 is fairly narrow and thus the voltage in the
secondary circuit 2 is correspondingly reduced. Under these
conditions the secondary circuit voltage gives only partial
alignment of the suspended particles in the cell 14, creating an
intermediate level of opacity in the cell 14 and an intermediate
level of reflectivity of the mirror as a whole.
If desired the ambient light detector 32 can be provided with a
time delay component (not shown in the Figures) so that the cell 14
is not returned prematurely to the clear condition by the lights of
a passing vehicle which briefly raise to a high level the ambient
light reaching the detector 32.
In a typical example of a control system according to the invention
the cell 14 had a capacity of 11 nF and the condenser 16, formed of
four 22 nF condensers connected in series, thus had a capacity of
5.5 nF. Each E-shaped ferrite measured 25 mm (height).times.13 mm
(width).times.8 mm (depth) and was made of material 3C8. The
induction coil had 66 turns in the primary circuit (1) on one
portion of the core, 240 turns in the secondary circuit (2) on
another portion of the core, and a gap of 2.5 mm in the magnetic
circuit. The primary winding 11 had an inductance L.sub.p of 0.318
mH, the secondary winding 12 had an inductance L.sub.s of 6.31 mH
and the mutual inductance M was 0.6 mH. The coupling coefficient K
of the induction coil, calculated by the formula quoted above, was
0.423.
In a variation of the above arrangement the secondary circuit can
be part of an auto-oscillating circuit. In this alternative the
secondary (resonant) circuit imposes the operational frequency on
the oscillator.
* * * * *